This application claims benefit of priority to Japanese Application No. JP2002-45155 filed Feb. 21, 2002, the entire content of which is incorporated by reference herein.
1. Field of the Invention
The present invention relates to a method of controlling a motor that may suitably be applied to a reluctance motor, permanent magnet reluctance motor of a design with a high reluctance torque ratio or to an embedded magnet motor etc. and to a device therefor.
2. Description of the Related Art
Conventionally, when controlling the output torque of a permanent magnet motor or reluctance motor with high precision at high speed, it was necessary to provide a rotor position sensor for supplying current corresponding to the position of the motor rotor.
However, since a rotor position sensor is of comparatively large volume, it invites restrictions regarding arrangement, and causes of failure such as difficulty of arranging control transmission wiring for transmitting the sensor output to the control device, or disconnection etc are increased.
In this regard, in the case of a permanent magnet motor, it is possible to ascertain the rotor position indirectly by detecting the motor back e.m.f. (that is to say, back electromotive force) generated during rotation due to the permanent magnet magnetic flux. However, in the case of a reluctance motor, where no back e.m.f. is generated by the permanent magnet and also in the case of a reluctance torque/permanent magnet motor, the motor back e.m.f. is relatively small in a motor of a design in which the ratio of the reluctance torque with respect to the permanent magnet torque is high, so the motor back e.m.f. cannot be accurately detected. It was therefore not possible to ascertain the rotor position.
FIG. 1 shows an example of the construction of a control block of a motor control device in which control is effected by inferring the rotor position of a permanent magnet motor without using a rotor position sensor, as was done conventionally.
As shown in FIG. 1, a motor control device 10 controls a PWM inverter 40 that drives a permanent magnet motor 30 and comprises a dq current pattern setting section (dqCPSS) 11, current control section (CCS) 12, voltage co-ordinate conversion section (VCCS) 13, current co-ordinate conversion section (CCCS) 15, rotor position inferring section (RPIS) 18 and d voltage inferring section (dVIS) 19.
In this motor control device 10, the permanent magnet flux direction is defined as the dr axis direction. If the direction orthogonal thereto is taken as the qr axis, the back e.m.f. (that is to say, back electromotive force) generated by rotation of the permanent magnet motor 30 is generated only in the qr axis direction.
Consequently, if the interred orthogonal co-ordinates of the rotor position are defined as the dr and qr axes and the inferred angle of the rotor position is successively corrected such that the induced voltage in the dr axial direction becomes zero, for the true d and q axes, the d axis coincides with the dr axis and the q axis coincides with the qr axis, so the true rotor position can be inferred.
However, in recent years, permanent magnet reluctance motors and embedded magnet motors which are designed to have a large reluctance torque ratio have started to be used in applications such as electric automobiles.
When the conventional method of inferring rotor position described above is applied to such motors, in some conditions it may be impossible to infer the rotor position accurately.
For example, in the case of a motor whose motor equivalent circuit constants are Ld=181 xcexcH, Lq=466 xcexcH, "PHgr" PM=0.068 Wb (where Ld and Lq are the d and q axis inductances and "PHgr" PM is the permanent magnet flux), taking the deviation between the inferred rotor phase xcex8H and the true rotor phase xcex8 along the horizontal axis, FIG. 4B shows the results of measuring the calculated value xcex94Vd of the induced d axis voltage for various phase deviations.
In rotor position inference in a conventional motor control device, the rotor position is inferred by successively correcting the inferred value of the rotor position such that xcex94Vd becomes zero.
As shown in FIG. 4B, xcex94Vd is assigned a value that is substantially close to zero when the deviation between the inferred rotor phase xcex8H and the true rotor phase xcex8 is negative.
The reason for this is that, if the conventional control system is employed, even though the inferred rotor phase departs greatly from the true phase, since xcex94Vd is close to 0, it is assumed that a substantially correct position has been inferred and correction is therefore discontinued. As a result, the correct rotor position cannot be inferred.
Accordingly, in a motor such as a reluctance motor, permanent magnet reluctance motor designed with a high reluctance torque ratio, or embedded magnet motor, one object of the present invention is to provide a novel method of controlling a motor capable of controlling output torque with high precision and high speed whereby the rotor position can be accurately inferred without providing a rotor position sensor, and a device therefor.
In order to achieve the above object, the present invention is constituted as follows. Specifically, in a method of controlling a reluctance torque/permanent magnet motor comprising a rotor having protuberant magnetic polarity, if the inferred axis of the rotor permanent magnet flux direction is defined as the dr axis, the direction orthogonal to this dr axis as the qr axis, and an axis offset by a predetermined angle from said dr axis as the xcex3 axis, on this xcex3 axis, the xcex3 axis inferred voltage value which is inferred and calculated using the detected value of the motor current and motor equivalent circuit constant and the xcex3 axis component voltage applied to said motor are compared, and the dr axis phase angle is inferred and calculated such that the deviation thereof is substantially zero.